In certain applications metal-oxide oxygen carriers are utilized for the delivery of oxygen via reduction. One such application which has been investigated extensively is chemical looping combustion. Chemical looping combustion systems generally utilize a fuel reactor, an air reactor, and a metal oxide oxygen carrier undergoing reduction in the fuel reactor and oxidation in the air reactor. The reduction in the fuel reactor is facilitated by close contact between a fuel and the oxygen carrier. The subsequent oxidation of the carrier in the air reactor is an exothermic process, and a stream of N2 is exhausted from the air reactor and carries the heat of oxidation to an attached power generation island.
Chemical looping combustion cycles provide potentially significant advantages. The enhanced reversibility of the two redox reactions offers improved efficiencies over traditional single stage combustions, where the release of a fuel's energy occurs in a highly irreversible manner. Further, with appropriate oxygen carriers, both redox reactions can occur at relatively low temperatures, allowing a power station to more closely approach an ideal work output without exposing components to excessive working temperatures. Additionally, and significantly, chemical looping combustion can serve as an effective carbon capture technique. Of the two flue gas streams generated, one is comprised of atmospheric N2 and residual O2, but sensibly free of CO2, while the second stream is comprised of CO2 and H2O, and contains almost all of the CO2 generated by the system. It is relatively uncomplicated to remove the water vapor, leading to a stream of almost pure CO2. For these reasons, chemical looping combustion systems have been extensively investigated. However, necessary characteristics of the oxygen carrier such as sufficient durability and reactivity have limited the success, particularly when the fuel utilized has been introduced to the fuel reactor as a solid such as carbon, coal, or biomass.
Challenges associated with the chemical looping combustion of solid carbonaceous fuels include achieving sufficient combustion rates suitable for various reactor systems, sufficient oxygen release capacity facilitating the coal-oxygen carrier interactions, stable reactivity during multiple cycles, high attrition resistance, and low reactivity with ash and other contaminants. Additional issues arise when the solid carbonaceous fuel is introduced directly into a fuel reactor, without benefit of an initial gasification. Introduction of the solid carbonaceous fuel into the fuel reactor can generate a direct reduction of the oxygen carrier by carbon, and the combustion of solid carbonaceous fuels containing or subsequently generating significant amounts of a solid carbon component such as char is dramatically improved when an oxygen carrier capable of reduction from a solid-solid reaction with carbon is utilized. See e.g., Siriwardane et al., “Evaluation of reaction mechanism of coal-metal-oxide interactions in chemical looping combustion,” Combustion and Flame 157 (2010). Additionally, a significant concern may arise regarding mismatch between any gasifications within the reactor and the combustion rate at the same temperature. Such issues can prolong the residence time of coal inside the fuel reactor to fulfill the higher carbon conversion efficiency.
A variety of metal oxides have been evaluated for suitability as oxygen carriers in chemical looping combustion systems using solid fuels. Metal oxides based on Ni, Fe, Co, Cu and Mn have a good affinity with CO and are thermodynamically feasible as oxygen carriers, however, Mn2O3, Co3O4 and CuO decompose at relatively low temperatures. CuO has also been extensively investigated, but the low melting point and agglomeration issues introduce significant difficulties. Fe2O3 generally exhibits improved temperature stability, however the reactivity of Fe2O3 is significantly limited as compared to Cu-based oxygen carriers. Additionally, Fe2O3 requires relatively high temperatures as compared to CuO. These characteristics reduce overall system performance and increase the complexity of heat transfer requirements in a working system. Mixed systems have also been evaluated for solid fuels combustion in an attempt to optimize the characteristics of the individual constituents. See e.g., Wang et al., “Investigation of Chemical Looping Combustion of Coal with CuFe2O4 Oxygen Carrier,” Energy Fuels 25 (2011); see also U.S. patent application Ser. No. 13/159,553 by Siriwardane et al., filed Jun. 14, 2011. Iron (Fe) and manganese (Mn) compounds have also been investigated as oxygen carriers. See Shulman et al., “Manganese/Iron, Manganese/Nickel, and Manganese/Silicon Oxides Used in Chemical-Looping With Oxygen Uncoupling (CLOU) for Combustion of Methane,” Energy Fuels 23 (2009).
It would be advantageous to provide a metal ferrite oxygen carrier having improved durability and reactivity over metal oxides currently used in the chemical looping combustion of solid carbonaceous fuels, such as coal, coke, coal and biomass char, and the like. It would be additionally advantageous if the metal ferrite oxygen carriers exhibited improved reduction rates over typically used materials such as Fe2O3, and were comparable with CuO while avoiding the associated agglomeration issues. It would be particularly advantageous if the metal ferrite oxygen carriers were comparable in cost to Fe2O3 and could be prepared using readily available materials.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.